An optical disk unit passes write and read beams through an object lens, to immediately read (verify) a medium while writing the medium.
In this case, a lens actuator of the optical disk unit moves the write and read beams together to trace a track on the medium, and a galvanomirror drives the read beam to let it follow the track independently of the write beam. Position servo control of the read beam during the independent tracking operation is carried out according to a tracking error signal of the read beam. For suppressing unwanted movements of the galvanomirror due to disturbing vibration, position lock control for maintaining the galvanomirror at a central position is carried out according to a galvanomirror position signal.
Here, a zero point of the tracking error signal does not always agree with a zero point of the galvanomirror position signal. If the position servo control and position lock control are simultaneously carried out under this condition, opposite forces may be applied to the actuator due to the disagreement of the zero points, to produce a servo reminder (incomplete servo) and a track error after a track jump. It is required, therefore, to properly control the position of the read beam by effectively combining the position servo control according to the tracking error signal and the position lock servo control according to the galvanomirror position signal.
Tension of cables, etc., connected to the galvanomirror causes a stop point of the position lock control to deviate from a zero point of the galvanomirror position signal. Even if a servo gain of the galvanomirror is adjusted in a manufacturing stage such that the write and read beams are positioned on the same track, the gain may change through aging, to change the stop point of the read beam with the position locked galvanomirror during a seek operation. If this happens, the read beam may be positioned at a different track from a track of the write beam after the seek operation. Then, a corrective seek operation must be done for the read beam. It is necessary, therefore, to provide a read beam control system that can maintain an adjusted state of correctly positioning the read beam onto the same track as the write beam after the completion of a seek operation, even if the galvanomirror has a mechanical offset.
If a servo system malfunctions during the tracking control, the galvanomirror will be abnormally deflected to zero the tracking error signal to indicate that there is no track error. In this case, the servo error will never be detected. It is required, therefore, to detect the abnormal deflection of the galvanomirror and carry out a proper restoration process.
The tracking error signal obtained from the read beam in response to the position of the galvanomirror may have an offset, which may cause the read beam to be off-track. It is required, therefore, to remove the offset depending on the position of the galvanomirror.